1. Field of the Invention
The need for curved glass sheets has increased tremendously in recent years, particularly with the increase in use of curved glass for windshields, sidelights and rear windows of automobiles. The problem of increasing the rate of production of shaped glass sheets on a mass production line has been recognized and the glass shaping art developed new methods and apparatus that do not necessarily require sheets to be stopped at a shaping station as in U.S. Pat. No. 3,374,080 to Robert W. Wheeler or in U.S. Pat. No. 3,468,645 to Harold A. McMaster et al. The roll forming method developed in response to this need is disclosed in U.S. Pat. No. 3,701,644 to Robert G. Frank. The present invention relates to an improvement in the roll forming apparatus of the aforesaid patent.
U.S. Pat. No. 3,701,644 to Robert G. Frank discloses a method and apparatus for shaping sheets of heat-softenable material, such as glass sheets, with opposed sets of rotating forming rolls. The sheets of glass or other softenable material are shaped while moving between the rotating rolls. This technique for shaping glass sheets has been identified by the term "roll forming."
Usually, the glass sheets to be shaped by roll forming are conveyed as a series of heat-softened sheets along a roller conveyor in a flat state into the roll forming station. The rolls of the roller conveyor are spaced longitudinally in a common upper tangential plane that defines the path of movement of the glass sheets into the roll forming station. According to the roll forming method, each heat-softened flat glass sheet is engaged between an upper set and a lower set of rotating forming rolls of complementary shape. Each set is carried by a movable roll housing. When each glass sheet in succession reaches a position wherein its entire length is between the forming rolls, the lower set of forming rolls lifts the glass sheet to a position above the conveyor where the sets of rotating forming rolls engage the opposite surfaces of the glass sheet for sufficient time to impress the shape of the forming rolls on the glass sheet as the latter passes between the sets of rotating forming rolls. The lower set of rotating forming rolls is lowered to redeposit the shaped glass sheet on the conveyor.
The shaped sheet proceeds along the conveyor to a cooling or quenching station where the glass sheet is cooled at a rate sufficient to impart either a partial or a full temper or an anneal depending upon the rate of cooling. However, if the glass sheet is redeposited on a rigid conveyor roll of cylindrical configuration after it is shaped, the glass sheet tends to become somewhat flat before it reaches the cooling station.
In the roll forming method, it is essential that the glass be formed as closely as possible to the ultimate shape desired. The glass sheet arrives at the roll forming station in a deformable condition but is usually still essentially flat when it arrives, and leaves the roll forming station in a curved configuration. It is desired that the glass not be prematurely shaped to a wrong shape which has to be pressed out by the rotating forming rolls and it is also desired that the glass sheet after being shaped by roll forming not lose its shape while it is still hot enough to be deformed and before it is cooled at the cooling station after its passage through the roll forming station.
Flexible conveyor rolls comprising spaced stub shafts drivingly interconnected by springs that assume the shape of bent glass sheets for use in a glass shaping station are disclosed in U.S. Pat. No. 3,485,615 to Rahrig et al. and U.S. Pat. No. 3,485,618 to Ritter. In both of these patents a flexible spring is the only member secured at its opposite ends to a pair of spaced stub rolls to transmit a rotating driving force from one stub roll to the other. Whenever the bearing for the driven stub roll binds, the spring interconnecting the stub rolls develops a torsion force that causes it to develop torsional vibration. The torsional vibration causes the glass to move in a zig-zag pattern rather than along a straight line. If the glass sheet arrives at the cooling station in a skewed orientation, the blasts of cool tempering medium that chill the glass tend to distort the shape of the glass. Furthermore, when a stub shaft driven through a spring as in Ritter freezes to its bearing housing, the connecting spring winds into tighter and tighter coils until it breaks upon reaching the elastic limit. In Rahrig et al., where each stub shaft is driven independently, it is necessary to drive both stub shafts at the same peripheral speed to avoid failure of the connecting spring.
The flexible conveyor rolls of the present invention which transport the bent glass sheets out of the roll forming station are constructed to avoid these problems.
Furthermore, all the conveyor rolls at the glass shaping station of the latter two patents are flexible. Flexible rolls of the type disclosed in these patents contain many components and are more difficult to maintain in good operating condition than rigid rolls.